Iron- and copper-comprising heterogeneous catalyst and process for preparing olefins by reacting carbon monoxide with hydrogen

ABSTRACT

Iron- and copper-containing heterogeneous catalyst, and a process for producing it, including the following steps: thermal decomposition of gaseous iron pentacarbonyl to give carbonyl iron powder having spherical primary particles; treatment of carbonyl iron powder with hydrogen, resulting in the metallic spherical primary particles at least partly agglomerating; surface oxidation of the iron particles to form iron oxide; contacting the particles with an aqueous solution of a copper compound; drying in the presence of oxygen and subsequent calcination in the absence of oxygen, resulting in oxygen-comprising copper compounds on the particles; and finally reaction of these with the iron oxide to form a mixed oxide of the formula Cu x Fe 3-x O 4 , where 0&lt;x≦1. Process for preparing olefins by reacting carbon monoxide with hydrogen in the presence of a catalyst, wherein the abovementioned iron- and copper-comprising heterogeneous catalyst is used as catalyst.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of European patent application no.09175224.6 filed Nov. 6, 2009, the contents of which are incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an iron- and copper-comprisingheterogeneous catalyst, a process for producing it and a process forpreparing olefins by reacting carbon monoxide with hydrogen in thepresence of the iron- and copper-comprising heterogeneous catalyst.

BACKGROUND OF THE INVENTION

It is known that lower olefins can be prepared from carbon monoxide (CO)and hydrogen (H₂) over metal catalysts, e.g. iron or cobalt catalysts.Iron oxides are usually used as catalyst precursors. Such catalysts aredescribed, for example, in U.S. Pat. No. 4,544,674, U.S. Pat. No.5,100,856, U.S. Pat. No. 5,118,715, U.S. Pat. No. 5,248,701, US2004/0127582 A1, H. P. Withers et al., Ind. Eng. Chem. Res. 1990, 29,pages 1807 to 1814, and M. E. Dry et al., Stud. Surf. Sci. Catal., Vol.152, 2004, pages 533 to 600.

This reaction is also referred to as the Fischer-Tropsch synthesis.

Conventional processes for the Fischer-Tropsch synthesis producehydrocarbons having a wide product distribution.

In principle, this range of the product distribution can becharacterized by the Anderson-Schulz-Flory distribution; cf.: M.Janardanarao, Ind. Eng. Chem. Res. 1990, 29, pages 1735-53.

It is likewise known that the composition of the hydrocarbons formed inthe Fischer-Tropsch process can be strongly influenced by the choice ofcatalysts used, the types of reactor and the reaction conditions.

For example, it is known that the product distribution can be shifted inthe direction of lower olefins by use of high temperatures in thepresence of modified iron catalysts: B. Büssemeier et al., HydrocarbonProcessing, November 1976, pages 105 to 112.

The main problem here is the formation of large amounts of undesirablemethane (CH₄).

In addition, the iron oxides required as starting material for thecatalyst are difficult to reduce.

DE 28 22 656 A1 (Inst. Fr. du Petrole) discloses a Fischer-Tropschprocess in which the catalyst is obtained by precipitation of ametal-organic iron and/or cobalt and/or nickel aggregate on an inorganicsupport. The precipitation of the aggregate on the support is effectedby impregnating the support with a solution of the aggregate.C2-C4-Olefins (“lower olefins”) and only small amounts of methane aresaid to be formed selectively in this process. The main disadvantage ofthese catalysts is that the active catalyst constituents can be volatileunder the reaction conditions, which results in a loss of metal, andthat they are toxic.

DE 29 19 921 A1 (Vielstich et al.) describes a further Fischer-Tropschprocess in which catalysts comprising polycrystalline iron whiskers assubstantial catalyst component are used. These iron whiskers areobtained by thermal decomposition of iron pentacarbonyl in a magneticfield. The iron whiskers are preferably used as pellets. According tothe teachings of this DE document, polycrystalline whiskers are fineiron threads having microscopically small single crystal regions (page5, 3rd paragraph). The shape of the thread-like primary particlesresults from growth in the magnetic field. The threads have a length of,for example, from 0.06 to 1 mm.

The two figures in “Fachberichte für Oberflächentechnik”, July/August1970, page 146, show scanning electron micrographs of such a carbonyliron powder having thread-like primary particles.

“Fachberichte für Oberflächentechnik”, July/August 1970, pages 145 to150, also describes these iron whiskers as metal hairs which result fromcrystal growth of the metal in thread form, unlike normal crystal growth(page 145, 2nd paragraph). In the polycrystalline iron whiskers, theratio of length to diameter is, for example, ≧10.

Such polycrystalline iron whiskers are also described in H. G. F.Wilsdorf et al., Z. Metallkde. 69 (11), 1978, pages 701 to 705.

DE 25 07 647 A1 (Kölbel et al.) describes the use of catalystscomprising manganese and optionally iron for preparing hydrocarbons andoxygen-comprising compounds from CO and H₂.

U.S. Pat. No. 2,417,164 (Standard Oil Comp.) relates to processes forsynthesizing liquid hydrocarbons from CO and H₂ in the presence of metalcatalysts, including carbonyl iron powder.

WO 07/060,186 A1 (BASF AG) teaches processes for preparing olefins fromsynthesis gas using Fischer-Tropsch catalysts in a reaction column.

WO 09/013,174 A2 (BASF SE) relates to a process for preparingshort-chain, gaseous olefins by reacting carbon monoxide with hydrogenin the presence of an iron-comprising heterogeneous catalyst, wherecarbonyl iron powder having spherical primary particles is used ascatalyst.

Promoters in iron catalysts for Fischer-Tropsch syntheses are described,for example, in the abovementioned WO 09/013,174 A2 and in M.Janardanarao, Ind. Eng. Chem. Res. 1990, 29, pages 1735 to 1753, and C.D. Frohning et al. in “Chemierohstoffe aus Kohle”, 1977, pages 219 to299.

As suitable promoters, the catalyst can comprise, for example, one ormore of the elements potassium, vanadium, copper, nickel, cobalt,manganese, chromium, zinc, silver, gold, calcium, sodium, lithium,cesium, platinum, palladium, ruthenium, sulfur, in each case inelemental form or in ionic form.

EP patent application no. 08164085.6 (BASF SE) of Sep. 10, 2008describes an integrated process in which pure carbonyl iron powder (CIP)is prepared by decomposition of pure iron pentacarbonyl (IPC) in a plantA, in which carbon monoxide (CO) liberated in the decomposition of theIPC is used for preparing further CIP from iron in plant A or is fed toan associated plant B for preparing synthesis gas or is fed to anassociated plant C for preparing hydrocarbons from synthesis gas and theCIP prepared in plant A is used as catalyst or catalyst component in anassociated plant C for preparing hydrocarbons from synthesis gas fromplant B.

Two parallel European patent applications having the same filing date(all BASF SE) relate to particular iron-comprising heterogeneouscatalysts and their use in processes for preparing olefins by reactingcarbon monoxide with hydrogen.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the disadvantagesof the prior art and discover an improved catalyst and an improvedeconomical process for preparing olefins. The process should, inparticular, give lower olefins (e.g. C2-C6-olefins, in particularC2-C4-olefins), in particular ethene, propene and 1-butene, veryselectively while at the same time forming very small amounts ofmethane, carbon dioxide, alkanes (e.g. C2-C6-alkanes, in particularC2-C4-alkanes) and higher hydrocarbons, i.e. hydrocarbons having, forexample, seven or more carbon atoms (C7+ fraction), in particular fiveor more carbon atoms (C5+ fraction). Constituents of the catalyst shouldnot be volatile under the reaction conditions.

Furthermore, the catalyst should have a shortened activation phase. Therunning-in time until the desired product spectrum is achieved, which isknown for the Fischer-Tropsch synthesis, should be shortened.

The catalyst should have an improved operation life and increasedmechanical stability. The increased stability is, in particular,advantageous when the catalyst is used in a fluidized bed or in slurryreactors or else in bubble columns.

According to the invention, the following aspects, inter alia, wererecognized:

The metallic secondary particles formed with at least partialagglomeration in step II, particularly in a fluidizable fraction havingparticle diameters in the range 10-250 μm (see below), are idealcatalyst precursors for the synthesis of lower olefins from CO-richsynthesis gases because of their chemical composition. An additionaladvantage is the low surface area of the particles, which is preferablybelow 2 m²/g (see below).

A particular advantage is the low oxygen content of the metallicsecondary particles, as a result of which a reduction, and thusactivation of the catalyst, is greatly simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 show scanning electron micrographs of preferred carbonyl ironpowder having spherical primary particles before the hydrogen treatmentaccording to step II in the disclosure.

FIGS. 4-5 show, by way of example, agglomerates obtained after thehydrogen treatment.

FIG. 6 shows a carbonyl iron powder filled by optional treatment withiron pentacarbonyl with a depiction of the pore structure.

FIG. 7 show the pore distribution after step II and after step IIb.

We have accordingly found an iron- and copper-comprising heterogeneouscatalyst and a process for producing it, which comprises the followingsteps:

I. thermal decomposition of gaseous iron pentacarbonyl to give carbonyliron powder having spherical primary particles,

II. treatment of carbonyl iron powder obtained in step I with hydrogen,resulting in the metallic spherical primary particles at least partlyagglomerating,

III. surface oxidation of the iron particles from step II (i.e.agglomerates=secondary particles, and also any primary particles stillpresent) to form iron oxide,

IV. contacting of the particles from step III with an aqueous solutionof a copper compound,

V. drying in the presence of oxygen and subsequent calcination in theabsence of oxygen, resulting firstly in oxygen-comprising coppercompounds on the particles and finally reaction of these with the ironoxide to form a mixed oxide of the formula Cu_(x)Fe_(3-x)O₄, where0<x≦1.

Furthermore, we have accordingly found a process for preparing olefinsby reacting carbon monoxide with hydrogen in the presence of a catalyst,wherein the abovementioned iron- and copper-comprising heterogeneouscatalyst is used as catalyst.

The proportion of spherical primary particles in the carbonyl ironpowder obtained in step I is preferably >90% by weight, particularlypreferably >95% by weight, very particularly preferably >98% by weight.

The spherical primary particles obtained in step I preferably have adiameter in the range from 0.01 to 50 μm, particularly preferably in therange from 0.1 to 20 μm, very particularly preferably in the range from0.5 to 15 μm, more particularly in the range from 0.7 to 10 μm, moreparticularly in the range from 1 to 10 μm.

The iron content of the spherical primary particles is preferably >97%by weight, particularly preferably ≧99% by weight, in particular ≧99.5%by weight.

The iron is preferably present in its most thermodynamically stablemodification (alpha-iron).

The spherical primary particles are preferably free of pores.

In particular, the carbonyl iron powder has no thread-like primaryparticles in addition to the spherical primary particles, especially notthe iron whiskers disclosed in DE-A1-29 19 921 and “Fachberichte fürOberflächentechnik”, July/August 1970, pages 145 to 150 (see above).

FIGS. 1 to 3 show scanning electron micrographs of preferred carbonyliron powder having spherical primary particles before the hydrogentreatment according to step II.

Carbonyl iron powder having spherical primary particles which can beused in the process of the invention is, for example, obtainable underthe trade name “carbonyl iron powder CN” from BASF AG or now BASF SE,D-67056 Ludwigshafen.

The carbonyl iron powder having spherical primary particles is obtainedby thermal decomposition of gaseous iron pentacarbonyl (Fe[CO]₅), whichhas particularly preferably been purified beforehand by distillation.

The product obtained in step I is treated with hydrogen in step II. Thistreatment of the primary particles with hydrogen is preferably carriedout at a temperature in the range from 300 to 600° C. This treatmentlowers the residual content of carbon, nitrogen and oxygen in the CIP.(DE 528 463 C1, 1927). Here, the spherical primary particles are atleast partly, e.g. to an extent of from 25 to 95% by weight,agglomerated.

The metallic secondary particles formed with at least partialagglomeration in step II preferably have particle diameters in the rangefrom 10 to 250 μm, particularly preferably from 50 to 150 μm. Suchfluidizable particle fractions can be obtained by appropriate sieving.

In step II, metallic secondary particles having BET surface areas (DINISO 9277) of preferably less than 2 m²/g, in particular from 0.2 to 1.9m²/g, are formed.

FIGS. 4 and 5 show, by way of example, agglomerates obtained after thehydrogen treatment.

In step III, the iron particles from step II (i.e. theagglomerates=secondary particles, and also any primary particles stillpresent) are subjected to controlled surface oxidation (passivated). Inthis oxidation, iron oxide is formed on the surface of the particles.The oxidation, also referred to as passivation, is preferably carriedout by means of oxygen. The oxygen can be used in the form ofoxygen-comprising (O₂-comprising) water.

The oxidation is preferably carried out at temperatures below 150° C.,particularly preferably at a temperature of less than 50° C., inparticular at a temperature in the range from 20 to 45° C., e.g. in airdiluted with inert gas, oxygen-comprising inert gas or by bringing theparticles into contact with oxygen-comprising water, in this casepreferably with stirring. Suitable inert gases are nitrogen or noblegases such as He, Ne, in particular argon.

In step IV, the surface-oxidized (passivated) particles are preferablybrought into contact with an aqueous copper salt solution, in particularan aqueous solution of copper nitrate, copper carbonate or an organiccopper salt; this is also referred to as impregnation or steeping.

In a particular embodiment, compounds which reduce the surface tensionof the impregnation solution, e.g. surfactants, can be added to theaqueous copper salt solutions.

A particularly preferred copper salt is copper nitrate. Examples oforganic copper salts are copper acetate, copper oxalate and copperacetylacetonate.

In step V, drying is carried out in the presence of oxygen, preferablyin air, in particular at a temperature in the range from 50 to 150° C.,preferably from 55 to 120° C.

Calcination in the absence of oxygen (O₂), preferably under inert gas(i.e. in an inert gas atmosphere), particularly preferably at atemperature in the range from 500 to 800° C., in particular from 600 to750° C., is subsequently carried out. Suitable inert gases do not reactwith the iron and the dopants under the conditions and are, for example,noble gases such as He, Ne, in particular Ar.

Here, mixed oxides, in particular spinels, of the formulaCu_(x)Fe_(3-x)O₄, where x is in the range from >0 to ≦1, in particularin the range from >0.25 to ≦1, are formed by reaction of theoxygen-comprising copper compounds formed with iron oxide.

The doping of the catalyst obtained in step V with Cu is preferably inthe range from 0.5 to 4% by weight, particularly preferably from 0.6 to2% by weight, very particularly preferably from 0.7 to 1.5% by weight,e.g. from 0.8 to 1.3% by weight, in each case based on iron and in eachcase calculated as element in the oxidation state 0.

In a particular variant, the particles are additionally doped with atotal amount in the range from 0.01 to 1% by weight, particularlypreferably from 0.05 to 0.5% by weight, (in each case based on iron andin each case calculated as element in the oxidation state 0) of alkalinemetal ions and/or alkaline earth metal ions, in particular potassiumions and/or sodium ions.

This additional doping is, in particular, carried out after step V bycontacting, more particularly in step V between drying and calcinationby contacting, preferably in step IV by preferably simultaneouscontacting of the particles (secondary particles and any primaryparticles still present) with an aqueous solution of an alkali metalcompound and/or alkaline earth metal compound.

In a particular embodiment of the invention, steps IIa and IIb below areadditionally carried out between steps II and III.

In step IIa, the agglomerates are preferably brought into contact withliquid or gaseous iron pentacarbonyl. Particular preference is given toliquid iron pentacarbonyl. For this purpose, the metal secondaryparticles are, for example, introduced into a vessel made inert by meansof argon and dried at elevated temperature, e.g. from 70 to 150° C., inparticular, for example, at an internal temperature of the vessel of105° C. Iron pentacarbonyl is then introduced in liquid form in portions(e.g. of 5% by volume based on the amount of carbonyl iron powder), e.g.through an inlet tube.

The alternative contacting with gaseous iron pentacarbonyl can, forexample, be carried out in a fluidized bed, in particular at atemperature in the range from 120 to 175° C. It is preferably carriedout at an IPC partial pressure (absolute) in the range from 0.7 to 1bar.

In step IIb, iron pentacarbonyl is thermally decomposed, preferably at atemperature in the range from 150 to 350° C., in particular in the rangefrom 150 to 200° C. For example, the vessel in which the material fromstep III is present is heated to an internal temperature in the range ofpreferably from 150 to 180° C. and the decomposition reaction of the IPCapplied is preferably monitored using an IR spectrometer. When the COcontent of the offgas has passed its maximum, the vessel is cooled againto, for example, 105° C.

Depending on the desired degree of fill of the pores, the procedure ofthe two steps IIa and IIb is repeated.

Step IIb results in largely pore- and void-free secondary particles. Thesecondary particles obtained in step II comprise interstitial poresbetween the spherical primary particles (pore diameter, in particular,<4000 nm). The interstitial pores, in particular the interstitial poreshaving diameters of <4000 nm, thus represent intraparticulate pores(FIGS. 5 and 7), while the measured pores having diameters of, inparticular, >4000 nm can be interpreted as interparticulate pores(resulting from the interstitial volume between the secondaryparticles).

The treatment of the secondary particles with iron pentacarbonyl makesit possible to fill the interstitial pores between the spherical primaryparticles, which have, in particular, pore diameters in the range of<4000 nm. This thus gives predominantly pore- and void-free secondaryparticles in which the differential pore volume for pore diameters inthe range of <4000 nm is particularly preferably <10%, in a particularembodiment <5%, based on the measured integrated pore volume of thesecondary particles.

The amount of iron pentacarbonyl necessary for filling the pores havinga diameter of, in particular, <4000 nm is preferably determined by meansof pore volume measurement by mercury porosimetry (DIN 66133).

Particles obtained in step IIb are shown by way of example in FIG. 6.

The iron- and copper-comprising catalyst of the invention isparticularly preferably not applied to a support material.

In the process of the invention, the optionally doped iron- andcopper-comprising heterogeneous catalyst can be used in the form ofpellets.

The pellets are obtained by methods known to those skilled in the art.Preferred forms of the pellets are tablets and rings.

The pellets can also be comminuted again, e.g. by milling, before use inthe process of the invention.

The catalyst can be converted into a more synthesis-active state bytreatment with hydrogen and/or carbon monoxide at elevated temperature,in particular at temperatures above 300° C., before being used in theprocess of the invention. However, this additional activation is notabsolutely necessary.

In the process of the invention, the starting materials carbon monoxideand hydrogen are preferably used in the form of synthesis gas.

The synthesis gas can be produced by generally known processes (asdescribed, for example, in Weissermel et al., Industrial OrganicChemistry, Wiley-VCH, Weinheim, 2003, pages 15 to 24), for example byreaction of carbon or methane with steam or by partial oxidation ofmethane. The synthesis gas preferably has a molar ratio of carbonmonoxide to hydrogen in the range from 3:1 to 1:3. Particular preferenceis given to using a synthesis gas which has a molar mixing ratio ofcarbon monoxide to hydrogen in the range from 2:1 to 1:2.

In a particular embodiment of the process of the invention, thesynthesis gas comprises carbon dioxide (CO₂). The content of CO₂ ispreferably in the range from 1 to 50% by weight.

The process of the invention is preferably carried out at a temperaturein the range from 200 to 500° C., in particular from 300 to 400° C.

The absolute pressure is preferably in the range from 1 to 100 bar, inparticular from 5 to 50 bar.

The WHSV (weight hourly space velocity) is preferably in the range from100 to 10 000, particularly preferably from 300 to 5000, parts by volumeof feed stream per part by mass of catalyst and hour (l/kg·h).

Preferred reactors for carrying out the process of the invention are:fluidized-bed reactor, fixed-bed reactor, suspension reactor,microreactor.

In a fluidized-bed reactor, microreactor or suspension reactor, thecatalyst is preferably used in powder form.

The powder can also be obtained by milling previously produced pellets.

In a fixed-bed reactor, the catalyst is used as shaped bodies,preferably in the form of pellets.

The use of such reactors for the Fischer-Tropsch synthesis is described,for example, in C. D. Frohning et al. in “Chemierohstoffe aus Kohle”,1977, pages 219 to 299, or B. H. Davis, Topics in Catalysis, 2005, 32(3-4), pages 143 to 168.

The process of the invention gives a product mixture comprising olefinswith an olefin-carbon selectivity, in particular an α-olefin-carbonselectivity, for the C2-C4 range of preferably at least 30%, e.g. in therange from 30 to 50%. In the selectivity indicated, carbon dioxideformed is not taken into account (i.e. excluding CO₂).

In a particular embodiment, a product mixture comprising olefins isobtained with an olefin-carbon selectivity for the C2-C4 range of atleast 30%, e.g. in the range from 30 to 50%, with at least 90% of thisat least 30% being in turn made up of ethene, propene, 1-butene. Carbondioxide formed is not taken into account (i.e. excluding CO₂) in theselectivity indicated.

In a particularly preferred embodiment, a product mixture comprisingolefins is obtained with an olefin-carbon selectivity for the C2-C4range of at least 35%, e.g. in the range from 35 to 50%, with at least90% of this at least 35% being in turn made up of ethene propene,1-butene. Carbon dioxide formed is not taken into account (i.e.excluding CO₂) in the selectivity indicated.

The olefins obtained are used, for example, in processes for preparingpolyolefins, epoxides, oxo products, acrylonitriles, acrolein, styrene.See also: Weissermel et al., Industrial Organic Chemistry, Wiley-VCH,Weinheim, 2003, pages 145 to 192 and 267 to 312.

All pressures indicated are absolute pressures.

EXAMPLES Example 1 Comparative Catalyst Production of K—/Cu-DopedCarbonyl Iron Catalyst by Impregnation

100 g of carbonyl iron material having a particle size distribution ofthe secondary particles such that 90% by weight have a diameter in therange from 50 to 100 μm, see FIG. 4, were prepared from carbonyl ironpowder type CN, BASF AG or now BASF SE, by treatment with hydrogen atleast 300° C. and impregnated under ambient conditions (roomtemperature, atmospheric pressure) with 11 ml of aqueous potassiumnitrate/copper nitrate solution. The aqueous potassium nitrate/coppernitrate solution was produced by dissolving 3.87 g of copper nitrate(×2.5 H₂O, 99%, Riedel de Haen) and 0.66 g of potassium nitrate (99%,Riedel de Haen) in 11 ml of demineralized water. The impregnatedcatalyst was dried at 120° C. for 4 hours. The catalyst obtainedcomprised 0.23% by weight of K and 0.86% by weight of Cu.

Example 2 According to the Invention Production of K—/Cu-Doped CarbonylIron Catalyst (Mixed Oxide) by Impregnation

150 g of carbonyl iron material having a particle size distribution ofthe secondary particles such that 90% by weight have a diameter in therange from 50 to 100 μm, see FIG. 4, were prepared from carbonyl ironpowder type CN, BASF AG or now BASF SE, by treatment with hydrogen atleast 300° C. and passivated in a controlled manner by means of 5% byvolume of air in nitrogen in a rotary bulb oven at a temperature up tonot more than 35° C. The surface-passivated carbonyl iron powder wasimpregnated under ambient conditions (room temperature, atmosphericpressure) with 16.5 ml of aqueous potassium nitrate/copper nitratesolution. The aqueous potassium nitrate/copper nitrate solution wasproduced by dissolving 5.56 g of copper nitrate (×2.5 H₂O, 99%, Riedelde Haen) and 0.79 g of potassium nitrate (99%, Riedel de Haen) in 16.5ml of demineralized water. The impregnated catalyst was dried at 120° C.under a stream of air of 100 standard l/h for 4 hours in a rotary bulboven. The catalyst is subsequently calcined at 650° C. under a stream ofargon of 100 standard l/h for 10 hours.

The catalyst obtained comprised 0.17% by weight of K and 0.99% by weightof Cu.

Example 3 Filling of the Pores of Pure, Agglomerated Carbonyl IronPowder (Secondary Particles) from Step II with Iron PentacarbonylAccording to Steps IIa and IIb.

The amount of iron pentacarbonyl necessary for filling the pores havinga diameter of, in particular, <4000 nm was determined by means ofmercury porosimetry (DIN 66133).

200 ml of carbonyl iron material having a particle size distribution ofthe secondary particles such that 90% by weight have a diameter in therange from 50 to 100 μm, see FIG. 4, were prepared from carbonyl ironpowder type CN, BASF AG or now BASF SE, by treatment with hydrogen atleast 300° C. The carbonyl iron material was dried at 105° C. under anargon atmosphere in a stirred vessel for 5 hours. 10 ml of ironpentacarbonyl were then introduced. The vessel was subsequently heatedto an internal temperature of about 165° C. The decomposition occurredat 165° C. with stirring of the particles. The reaction was completewhen no iron pentacarbonyl or no free carbon monoxide was detected inthe offgas stream. These steps were repeated 13 times. After thesynthesis was complete, the product was flushed with argon at 100° C.for at least 12 hours until the CO or Fe(CO)₅ content of the offgas was<0.1 ppm by volume.

Comparison:

Performance of the Catalyst According to the Invention (Example 2) andthe Comparative Catalyst (Example 1) in the Process of the Inventionwith Prior Identical Activation

A series of comparative performance tests using in each case about 2.0 gof catalyst (examples 1, 2; WHSV=500 standard l/kg·h) and dilution withinert material (catalyst: silicon carbide=1:3 (weight ratio)) wascarried out. The catalysts were introduced into a fixed-bed reactor andpreactivated in H₂:N₂ (9:1) (molar) at 380° C. for 4 hours. Synthesisgas was then introduced into the reactor at a rate of about 0.9 standardl/h at 25 bar and the temperature was reduced to 340° C. As internalstandard for later analytical tests, 0.1 standard l/h of nitrogen gaswas additionally introduced. The results of the experiments carried outover a period of at least 75 hours are shown below for the respectivecatalyst systems.(Standard I=standard liters=volume converted to S.T.P., WHSV=weighthourly space velocity).

Example 2 (according to the Catalyst Example 1 invention) Synthesis gasratio 0.91 0.91 H₂/CO WHSV [standard l/kg · h] 500 500 % CO conversion97.5 96.1 % carbon selectivity to CH₄, 10.9 7.8 without CO₂ % carbonselectivity to C2-C4- 24.8 35.6 olefins, without CO₂ % carbon C7+,without CO₂ 40.9 27.2

Carbon dioxide formed is not taken into account (i.e. without CO₂) inthe selectivities indicated in the examples.

It can be seen that according to the invention methane formation isreduced and at the same time the yield of C2-C4-olefins is increased.

Analysis of the Reaction Products:

The product streams were sampled via heated stream selectors and linesafter the long-chain hydrocarbons had been condensed out in a hotseparator (about 160° C., 25 bar), and fed to an on-line gaschromatograph (GC).

GC: Agilent 6890N with FID and TCD.

Precolumns: CP-Poraplot Q, length 12.5 m, ID 0.53 mm, film thickness 20μm

FID:

Injector 250° C., split ratio 50:1, carrier gas helium, column DurabondDB-1 (length 60 m, ID 0.32 mm, film thickness 3 μm), detector 280° C.

TCD:

Injector 200° C., split ratio 10:1, carrier gas argon, column Carboxen1010 (length 30 m, ID 0.53 mm), detector 210° C.

Temperature program: 40° C.-5 min-7° C./min-250° C.-5 min, carrier gashelium.

FIGS. 1 to 3 below:

Carbonyl iron powder (CIP) having spherical primary particles which canbe used according to the invention in step II.

The invention claimed is:
 1. A process for producing an iron- andcopper-comprising heterogeneous catalyst, which comprises the followingsteps: I. thermal decomposition of gaseous iron pentacarbonyl to givecarbonyl iron powder having spherical primary particles, II. treatmentof carbonyl iron powder obtained in step I with hydrogen, resulting inthe metallic spherical primary particles at least partly agglomerating,III. surface oxidation of the iron particles from step II(agglomerates=secondary particles, and also any primary particles stillpresent) to form iron oxide, IV. contacting of the particles from stepIII with an aqueous solution of a copper compound, V. drying in thepresence of oxygen and subsequent calcination in the absence of oxygen,resulting firstly in oxygen-comprising copper compounds on the particlesand finally reaction of these with the iron oxide to form a mixed oxideof the formula Cu_(x)Fe_(3-x)O₄, where 0<x≦1.
 2. The process accordingto claim 1, wherein the oxidation in step III is carried out by means ofoxygen.
 3. The process according to claim 1, wherein the particles arebrought into contact with an aqueous copper salt solution in step IV. 4.The process according to claim 3, wherein the copper salt is coppernitrate or copper carbonate.
 5. The process according to claim 1,wherein the drying of the particles in step V is carried out at atemperature in the range from 50 to 150° C. and the calcination of theparticles is carried out at a temperature in the range from 500 to 800°C.
 6. The process according to claim 1, wherein the spherical primaryparticles obtained in step I have a diameter in the range from 0.01 to50 μm.
 7. The process according to claim 1, wherein the secondaryparticles (=agglomerates) used in step III have a diameter in the rangefrom 10 to 250 μM.
 8. The process according to claim 1, wherein theprimary particles obtained in step I have an iron content of greaterthan 97% by weight.
 9. The process according to claim 1, wherein theprimary particles obtained in step I are pore-free.
 10. The processaccording to claim 1, wherein the carbonyl iron powder obtained in stepI comprises no thread-like primary particles.
 11. The process accordingto claim 1, wherein the total doping of the catalyst obtained in step Vwith Cu is in the range from 0.5 to 4% by weight (based on iron). 12.The process according to claim 1, wherein the particles are additionallydoped with a total amount in the range from 0.01 to 1% by weight (basedon iron) of alkali metal ions and/or alkaline earth metal ions.
 13. Theprocess according to claim 12, wherein the doping in step IV is effectedby bringing the particles (secondary particles and any primary particlesstill present) into contact with an aqueous solution of an alkali metalcompound and/or alkaline earth metal compound.
 14. The process accordingto claim 1, wherein the particles are additionally doped with a totalamount in the range from 0.01 to 1% by weight (based on iron) ofpotassium ions and/or sodium ions.
 15. The process according to claim 1,which additionally comprises, between steps II and III, steps IIa andIIb below: IIa. contacting of the agglomerates (=secondary particles)with iron pentacarbonyl, IIb. thermal decomposition of the ironpentacarbonyl applied in step III to give at least predominantly pore-and void-free secondary particles.
 16. The process according to claim15, wherein the agglomerates are brought into contact with liquid orgaseous iron pentacarbonyl in step IIa.
 17. The process according toclaim 15, wherein the thermal decomposition of the iron pentacarbonyl instep IIb is carried out at a temperature in the range from 150 to 350°C.
 18. An iron- and copper-comprising heterogeneous catalyst obtained bythe process according to claim 1, wherein the at least partlyagglomerating metallic spherical primary particles of step IIagglomerate to form metallic secondary particles having BET surfaceareas of less than 2 m²/g, and wherein the drying in the presence ofoxygen of step V takes place at a temperature of from 50° C. to 150° C.19. A process for preparing olefins by reacting carbon monoxide withhydrogen in the presence of a catalyst, wherein the iron- andcopper-comprising heterogeneous catalyst according to claim 18 is usedas catalyst.
 20. The process according to claim 19, wherein the reactionis carried out at a temperature in the range from 200 to 500° C.
 21. Theprocess according to claim 19, wherein the reaction is carried out at anabsolute pressure in the range from 1 to 100 bar.
 22. The processaccording to claim 19 for preparing C2-C4-olefins.
 23. The processaccording to claim 19, wherein carbon monoxide and hydrogen are used inthe form of synthesis gas for the reaction.
 24. The process according toclaim 23, wherein the synthesis gas comprises carbon dioxide (CO₂). 25.The process according to claim 19, wherein carbon monoxide and hydrogenare used in a molar ratio in the range from 2:1 to 1:2.